The 3GPP work on standardization of the Long Term Evolution (LTE) concept is currently ongoing. In LTE the downlink will support channel dependent scheduling in both the time and frequency domains. The radio base station in the LTE network denoted evolved NodeB (eNodeB) will transmit reference signals that the mobile stations also termed User Equipments (UEs) use to determine the downlink channel quality. The UEs will send Channel Quality Indication (CQI) reports back to the eNodeB that are used by a scheduler located in the eNodeB for scheduling traffic within the cell that the eNodeB serves.
In the LTE network the UE is specified to be able to transmit different types of CQI reports. For example the UE will have the capability to transmit a full CQI reports and partial CQI reports. Full CQI report covers the whole downlink transmission bandwidth but they may have different frequency resolution. they may be filtered and processed in different ways, and they may be encoded in different ways, see also 3GPP TS 36.213 V8.2.0. “Physical layer procedures”.
Furthermore, CQI reports used together with Single Input Single Output (SISO), Multiple Input Single Output (MISO), Single Input Multiple Output (SIMO), or Multiple Input Multiple Output (MIMO) transmission could also be different for the different transmission cases. To exemplify, for MIMO a CQI report may e.g. include pre-coding weights to be used by the eNodeB multiple antenna transmission scheme.
The LTE uplink is based on single-carrier modulation and uses frequency division multiple access (FDMA) and time division multiple access (TDMA) principles. The LTE uplink consists of physical uplink control channels and data channels that are orthogonally frequency multiplexed. The single-carrier property of the LTE uplink makes it impossible for a UE to transmit on a physical control channel and a physical data channel in the same transmission-time-interval (TTI). Hence if a UE is transmitting data on a physical data channel the CQI that must be sent in the same TTI must also be sent on the physical data channel. When the CQI is transmitted on the physical data channel it is multiplexed with data. The UE will use the physical control channel to transmit CQI only in the case when the UE has no data transmission, and hence is not using the physical data channel.
The current assumption in 3GPP regarding the CQI reporting on the physical control channel is that a maximum of approximately 10 bits can be transmitted per UE and TTI. This number of bits leaves little space for fine granular CQI frequency domain information and MIMO information. On the physical data channel it will most likely be possible to transmit more bits.
When a UE is scheduled for data transmission on the physical channel it is signaled an uplink grant including resource block allocation and transport format selection. The uplink grant also includes 1 bit which indicates if a CQI report should be included in the uplink transmission or not. see R1-080597, Summary of AH on AI 6.3.5 “UE procedure for downlink shared channel”. The format of this CQI report is Radio Resource Control (RRC) configured.
When the eNodeB has transmitted data to the UE it receives a Hybrid Automatic Repeat Request (HARQ) Acknowledgement (ACK) or Negative Acknowledgement (NACK) from the UE. The eNodeB can for example keep track of the percentage of the number of downlink transmissions that are NACKed in the first transmission attempt, or any other predefined HARQ block error rate (BLER). It is often considered beneficial to keep the HARQ BLER at a certain target, for example 10%.
One way of using the uplink grant bit for requesting CQI is to set the grant bit when there is downlink data and a CQI report has not been received during a predetermined time T. It is however difficult to set the optimal time T. If T is set too large there will be a performance loss and if T is set too small the CQI report is transmitted unnecessarily often which results in a cost in terms of grant signaling in the downlink and uplink resources.
Hence, there exist a need for a method and a system that improve the procedure by means of which it is determined when CQI reports are transmitted from the UE.